Delay on-instant off with 555 timer

[Dannyx]

Good day folks. I'm trying to build a little circuit for a project of mine which requires pressing a button to toggle an output in the following fashion: if the button is held for around 5s, the output goes high. When the button is released, the output goes low instantly. If the button is released before the output actually goes high, the next press of the button should require 5s again before the output goes high again. This last bit is important, because what I was able to find was THIS circuit which uses the 555 timer to create a delayed-on circuit.

There's a couple of caveats I'm facing though: obviously you can see there's no button there - the countdown essentially starts when power is first applied to the whole circuit, so I'm not quire sure where I should drop the button. I guess I could cut VCC to the whole circuit using a SPDT. Even after the fact, I simulated this circuit and although it works, there's still the problem I mentioned earlier: if the button is released too soon, before the output actually swings high, the next press turns it on instantly, since the capacitor is still charged and pin 2, TR, is pulled to GND immediately. I cannot figure out how to overcome this...I tried coming up with various ways to "discharge" that cap so it'd be ready for a new cycle, but couldn't find one that works.

Most circuits I've seen work in the completely opposite way: the turn-on is instant and they SLOOWLY discharge the cap to get a delayed-OFF...

Any ideas on how to achieve such result ? Thanks.

EDIT: here's another idea I came up with and simulated and seems to work. I ditched the 555 entirely and used a MOSFET instead. I got the idea after watching THIS here video which coincidentally or not also talks about a delayed on>instant off scenario towards the end.

[budm]

Re: Delay on-instant off with 555 timer

How about this, using SPDT switch?

[Dannyx]

Re: Delay on-instant off with 555 timer

Thanks. So use the switch as a means to "reset" the FET ?

BTW, the simulator is HERE if you wish to mess around

[budm]

Re: Delay on-instant off with 555 timer

When SW is in N.C. (released), the cap is discharged through 100 Ohms so the relay will be off almost instantly.

[Dannyx]

Re: Delay on-instant off with 555 timer

What if I drop the resistor instantly and just short the cap out ? Arcing at the contacts ? I'm not going to do that, but I'm just curious.

[budm]

Re: Delay on-instant off with 555 timer

You can do that but it is not good practice. If you have good meter that can capture the discharge current then you can see what it is with and without resistor.

[petehall347]

Re: Delay on-instant off with 555 timer

will a diode work i wonder ?

[Dannyx]

Re: Delay on-instant off with 555 timer

I was thinking the same thing.

[budm]

Re: Delay on-instant off with 555 timer

Think about it, all we want is to discharge the caps down to basically 0V, so using diode the cap will not discharge to 0V because when the Voltage is <Vf of the diode it will no longer conduct, so basically the Voltage on the cap will go down to about 0.6V. So why do want to use diode, and what is the reason? The 50K is still in circuit so it will still keep on discharging the cap but it will take a long time to fully discharge charge the cap.

[Dannyx]

Re: Delay on-instant off with 555 timer

Just realized a time-delay relay would work just as well. Sure it's a bit bulky, but more fool-proof.

[R_J]

Re: Delay on-instant off with 555 timer

Just realized a time-delay relay would work just as well. Sure it's a bit bulky, but more fool-proof.

If thats the case maybe there is a solid state time delay relay that would work and be less bulky

[Curious.George]

Re: Delay on-instant off with 555 timer

If thats the case maybe there is a solid state time delay relay that would work and be less bulky

Or, a 25c microcontroller that lets you change your mind, later, as to how the circuit should operate in a variety of circumstances (e.g., is there a dead time that should be enforced after the cycle completes but before it can be triggered, again?)

[Dannyx]

Re: Delay on-instant off with 555 timer

Correct. That would work too for the "overkill" version, but would require a dedicated supply to run. The relay method COULD work straight off 230v. True, I haven't offered a description of the purpose of the build, that's why you guys are free to suggest any possible implementation of this idea, from simple to "scientific".

[eccerr0r]

Re: Delay on-instant off with 555 timer

Untested, I should just stick it in the simulator but too lazy.

[petehall347]

Re: Delay on-instant off with 555 timer

2 switches or rather one . and 2 relays .plus your timer circuit . will let you wire it up .

[eccerr0r]

Re: Delay on-instant off with 555 timer

Oh...btw, if you had a NC momentary switch, there is a trivial solution
I guess the assumption is that we had a NO switch.

[budm]

Re: Delay on-instant off with 555 timer

Yep, SPST N.C. will work too, N.C. will short out the timing cap until the sw goes open when pushed.

[Curious.George]

Re: Delay on-instant off with 555 timer

Correct. That would work too for the "overkill" version, but would require a dedicated supply to run.

Your "dedicated power supply" could be an LED and a 1/4W resistor. If an HE device, it might even light up to indicate "power supply ON". And, you can EXACTLY time those 5 seconds -- or 327 seconds (instead of relying on temperature compensated RC's to do the timing).

[Dannyx]

Re: Delay on-instant off with 555 timer

Untested, I should just stick it in the simulator but too lazy.

What simulator do you use for this ? Just curious.

[eccerr0r]

Re: Delay on-instant off with 555 timer

What simulator do you use for this ? Just curious.

For me, "Headspice"¹. You use whatever you want to simulate it.





¹ - simulated in my head.